Would you show things an Actor cannot do on a Use Case diagram?

Question

On a Use Case diagram can you show things that an actor cannot do, for example because they won't have permissions to do it?

Or is it just implied due to the fact that they won't have a line joining them to the particular use case?

Answer 1

No. An Actor would be connected to everything that he can do. If the Actor can't do it, then it's not shown.

Answer 2

If the Use Case you are diagramming is the case where an actor attempts to do something that is not permitted and is then denied, then yes, I would show it.

Otherwise, I would stick to only including things that are actually part of the use case.

Answer 3

You might model Role actors that can do the task. You could then have another use case that has the original actor attempting to acquire the given Role.

Answer 4

So from your answers are we saying that it depends on the 'system' that the use cases relate to?

i.e. If we are modelling a web site we would only show what different role Actors do have access to but if we were modelling the permission system behind the web site we could show the requests for permissions being denied?

Answer 5

This is what alternate paths are for. The basic path (a.k.a. happy path) will show what happens when the correct Actor initiates the Use Case. In the alternate paths you can show what happens if the wrong Actor attempts to initiate it.